h2 as uint8

This doesn't change the logic, but by defining h2 as uint8, and using
uint8 elsewhere, we can work properly with higher bits (and define empty
in it's binary form, instead of having to use the negative value and a
comment). This will also allow us future changes without having to play
with overflows.

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

bits_amd64.go

@@ -31,12 +31,12 @@ const (
 type bitset uint16
 
 func metaMatchH2(m *metadata, h h2) bitset {
-	b := simd.MatchMetadata((*[16]int8)(m), int8(h))
+	b := simd.MatchMetadata((*[groupSize]uint8)(m), uint8(h))
 	return bitset(b)
 }
 
 func metaMatchEmpty(m *metadata) bitset {
-	b := simd.MatchMetadata((*[16]int8)(m), empty)
+	b := simd.MatchMetadata((*[groupSize]uint8)(m), empty)
 	return bitset(b)
 }
 

bits_test.go

@@ -26,7 +26,7 @@ import (
 func TestMatchMetadata(t *testing.T) {
 	var meta metadata
 	for i := range meta {
-		meta[i] = int8(i)
+		meta[i] = uint8(i)
 	}
 	t.Run("metaMatchH2", func(t *testing.T) {
 		for _, x := range meta {
@@ -43,7 +43,7 @@ func TestMatchMetadata(t *testing.T) {
 			mask = metaMatchEmpty(&meta)
 			assert.NotZero(t, mask)
 			assert.Equal(t, uint32(i), nextMatch(&mask))
-			meta[i] = int8(i)
+			meta[i] = uint8(i)
 		}
 	})
 	t.Run("nextMatch", func(t *testing.T) {
@@ -54,7 +54,7 @@ func TestMatchMetadata(t *testing.T) {
 			assert.Equal(t, uint32(i), nextMatch(&mask))
 		}
 		for i := 0; i < len(meta); i += 2 {
-			meta[i] = int8(42)
+			meta[i] = uint8(42)
 		}
 		mask = metaMatchH2(&meta, h2(42))
 		for i := 0; i < len(meta); i += 2 {
@@ -66,7 +66,7 @@ func TestMatchMetadata(t *testing.T) {
 func BenchmarkMatchMetadata(b *testing.B) {
 	var meta metadata
 	for i := range meta {
-		meta[i] = int8(i)
+		meta[i] = uint8(i)
 	}
 	var mask bitset
 	for i := 0; i < b.N; i++ {
@@ -90,14 +90,13 @@ func nextPow2(x uint32) uint32 {
 }
 
 func TestConstants(t *testing.T) {
-	c1, c2 := empty, tombstone
-	assert.Equal(t, byte(0b1000_0000), byte(c1))
-	assert.Equal(t, byte(0b1000_0000), reinterpretCast(c1))
-	assert.Equal(t, byte(0b1111_1110), byte(c2))
-	assert.Equal(t, byte(0b1111_1110), reinterpretCast(c2))
+	assert.Equal(t, byte(0b1000_0000), empty)
+	assert.Equal(t, byte(0b1000_0000), reinterpretCast(empty))
+	assert.Equal(t, byte(0b1111_1110), tombstone)
+	assert.Equal(t, byte(0b1111_1110), reinterpretCast(tombstone))
 }
 
-func reinterpretCast(i int8) byte {
+func reinterpretCast(i uint8) byte {
 	return *(*byte)(unsafe.Pointer(&i))
 }
 

map.go

@@ -36,7 +36,7 @@ type Map[K comparable, V any] struct {
 // metadata is the h2 metadata array for a group.
 // find operations first probe the controls bytes
 // to filter candidates before matching keys
-type metadata [groupSize]int8
+type metadata [groupSize]uint8
 
 // group is a group of 16 key-value pairs
 type group[K comparable, V any] struct {
@@ -47,15 +47,15 @@ type group[K comparable, V any] struct {
 const (
 	h1Mask    uint64 = 0xffff_ffff_ffff_ff80
 	h2Mask    uint64 = 0x0000_0000_0000_007f
-	empty     int8   = -128 // 0b1000_0000
-	tombstone int8   = -2   // 0b1111_1110
+	empty     uint8  = 0b1000_0000
+	tombstone uint8  = 0b1111_1110
 )
 
 // h1 is a 57 bit hash prefix
 type h1 uint64
 
 // h2 is a 7 bit hash suffix
-type h2 int8
+type h2 uint8
 
 // NewMap constructs a Map.
 func NewMap[K comparable, V any](sz uint32) (m *Map[K, V]) {
@@ -150,7 +150,7 @@ func (m *Map[K, V]) Put(key K, value V) {
 			s := nextMatch(&matches)
 			m.groups[g].keys[s] = key
 			m.groups[g].values[s] = value
-			m.ctrl[g][s] = int8(lo)
+			m.ctrl[g][s] = uint8(lo)
 			m.resident++
 			return
 		}

simd/match.s

@@ -4,11 +4,11 @@
 
 #include "textflag.h"
 
-// func MatchMetadata(metadata *[16]int8, hash int8) uint16
+// func MatchMetadata(metadata *[16]uint8, hash uint8) uint16
 // Requires: SSE2, SSSE3
 TEXT ·MatchMetadata(SB), NOSPLIT, $0-18
 	MOVQ     metadata+0(FP), AX
-	MOVBLSX  hash+8(FP), CX
+	MOVBLZX  hash+8(FP), CX
 	MOVD     CX, X0
 	PXOR     X1, X1
 	PSHUFB   X1, X0

simd/src/asm.go

@@ -23,7 +23,7 @@ import (
 func main() {
 	ConstraintExpr("amd64")
 
-	TEXT("MatchMetadata", NOSPLIT, "func(metadata *[16]int8, hash int8) uint16")
+	TEXT("MatchMetadata", NOSPLIT, "func(metadata *[16]uint8, hash uint8) uint16")
 	Doc("MatchMetadata performs a 16-way probe of |metadata| using SSE instructions",
 		"nb: |metadata| must be an aligned pointer")
 	m := Mem{Base: Load(Param("metadata"), GP64())}